Airplane Tech Talk for the Non-Technical – Reciprocating Engines

Today is the first test of the quarter, and since I have utterly failed at my “write blog posts on the weekends and queue them up to prevent Tuesday-night blog-post-writing panic” plan, this post is going to be about airplane stuff. That’s interesting, right?

Last quarter we learned about the basics of turbine and reciprocating engines. The other day, I used the words “reciprocating” and “turbine” when talking to a friend, and quickly lost her. See, I thought I was being very clear, by saying “reciprocating” instead of “recip,” but apparently there’s more to explain. So, here’s everything you ever wanted to know about what makes airplanes move forward without falling out of the sky.

All airplanes, including gliders have lift, but flying on lift is like coasting in the air. You’ll go for a while, but eventually gravity will bring you back to earth. To stay in the air we need thrust. Thrust is provided by engines.

Most small, personal airplanes are powered by “recips.” Recips work through the reciprocating action of pistons going up and down inside cylinders. The pistons are moved when the fuel/air mixture that’s drawn into the cylinders (through various means, depending on the type of engine) is sparked and catches on fire.

The up and down motion is transferred through connecting rods to the crankshaft which turns it into rotary motion. (For the life of me, I can never type “crankshaft” correctly on the first go. It always comes out as “crankshart.” Make of that what you will.) Think about how an oil well pumps and you’ll have an idea how the crankshaft translates reciprocating motion into rotary motion.

There’s more to it, like how air and fuel get into the cylinders, and what order the air intake and combustion happen, and what order the cylinders fire. And the air gets compressed, either somewhat or a whole lot, depending on whether the engine is naturally aspirated or supercharged.

And sometimes turbos are involved. But the basic principle is pretty simple – set fuel on fire and translate that energy into motion, then translate one type of motion into another type of motion.

Oh, another thing? Recips are horribly inefficient. About 80% of the energy produced by the combustion of fuel goes out the exhaust. Bring on the electric cars!

I spent much of last week and the first part of this week wrestling with the concepts of fuel delivery. I won’t bore you – I literally fell asleep several times while reading about pressure carburetors and Bendix fuel injection systems. Suffice it to say, fuel is pumped in the engine, or pulled in by low pressure. One way or another it gets to the cylinders. Sometimes it’s delivered via little individual pipes and nozzles and sometimes it’s delivered wholesale to the intake manifold (which goes to the cylinders) via a carburetor barrel.

We say the words “nozzle” and “orifice” a lot in aviation. I take my entertainment where I can find it.

That covers recips. Next week I’ll talk about turbines … turbine engines are a big part of the reason I got interested in airplanes – they’re really cool and they’re fun to talk about! Till then, wish me luck on my test.